Fig. 99.—Tail skid with claw fitting.
Another version recently patented is to construct small planes to conform to the wing curve, and hinged so that by a system of wires and pulleys, actuated from the pilot’s seat, they could be adjusted to offer a normal surface to the direction of flight. The efficiency of this arrangement at low speeds is not very great, moreover a landing with the wind renders them quite useless. The best form of brake is undoubtedly one acting direct on the main wheels, either of the rim or band type, a good example of the latter being the system used on the 70 h.p. Bristol biplane. Closely allied to the question of brakes is that of steering, and the requirements of this latter item are fairly well satisfied by pivoting the tail skid and working it in conjunction with the rudder from the foot-bar or wheel.
Housing of Undercarriage during Flight.
Numerous suggestions, ideas, and patents exist, having as their object the housing of the undercarriage in the fuselage during flight, with a resultant reduction in resistance; and excellent as the principle is, its practical application is difficult of achievement—at least, for machines of the present. In flight the undercarriage is a useless encumbrance, adding weight and head resistance, so that an arrangement whereby this component could be folded into the main structure would apparently effect a saving in resistance. This would mean that the fuselage would be of larger cross-sectional area, the natural sequence being extra weight and resistance. It does not appear that the saving effected in resistance, when the undercarriage is folded during flight, would account for the additional weight of the operating mechanism and the increased head resistance of the fuselage, so that altogether the advantages of any so-called disappearing landing gear are very much more apparent than real. There is also the very great possibility of the undercarriage folding up or disappearing when it would be least required to do so. In the construction of the problematic air-liners of the future it may be possible to economically effect the housing of the undercarriage.
CHAPTER XII.
CONTROL SYSTEMS.
The mechanism by which the aeroplane is controlled in flight forms the connecting link between the pilot and machine, and constitutes a vitally important and somewhat vulnerable item of the complete structure.
Main Principles.
The control of all modern aeroplanes is effected in a lateral direction by small planes hinged to the rear spar of the outer ends of the wings, and known as “ailerons”; in a longitudinal or “fore-and-aft” direction by the elevator planes; and for steering by the rudder. Although these functions are alluded to separately, they are more often than not combined in their actions. The correct proportion of the controlling surfaces is an important factor in determining the ease or otherwise with which a machine can be handled in flight, and faults in this direction are responsible for the terms “heavy” or “stiff” on the controls being applied to a machine. The use of subsidiary flaps or ailerons for lateral control is a comparatively modern innovation. At one time it was usual to warp the entire plane, or in some cases the outer section only, and although the principle is the same—that of forming a negative or positive surface to the line of flight—structural considerations are wholly in favour of ailerons. With warping, the whole plane is subjected to continuous torsional movement, and to obtain this some of the trussing wires have necessarily to be arranged as control wires, the result being that the plane curvature loses its uniformity, and the whole girder system of the planes is less efficient under load than if the wires were permanently fixed; and the latter item is only possible with aileron control. Although it is usual to attach ailerons to both top and bottom planes of a biplane, there are occasions when sufficient control can be obtained with ailerons to the upper plane only, usually when the span of this plane is greater than that of the bottom.
Control by Inherent Stability.
With machines of the inherent stability class the lateral control is effected by additional means, the planes being designed to automatically right the effects of gusts. This element of inherent stability is obtained by suitably grading the camber and incidence of the wings, until at the wing tips the chord of the plane section forms a negative angle to the line of flight. Although this arrangement is undoubtedly of value, especially for the touring machine of moderate power, its chief fault lies in the relatively slow righting movements, which, although of no great consequence at a reasonable altitude, becomes a source of danger when alighting, and certainly entail the use of ailerons, or warp, to counteract it. The type was well exemplified in this country by the Handley-Page monoplane and biplane, while in Germany it achieved great popularity, surviving in some makes until the latter part of 1916. In the matter of control-surface design it is interesting to note the contrast between the preferences of English and German designers. In almost all German machines the ailerons, elevators, and rudder are balanced, i.e. surface is disposed each side of the hinge-axis, this applying to the small Albatross scouts and to the large machines of the Gotha class; while in this country few examples of this practice occur. The reason for the balancing of controls lies in the desire to reduce the manual strain on the pilot to a minimum; and it appears that with large machines balanced surfaces will be imperative. Several automatic controls have been produced, the most notable perhaps being the Sperry gyroscopic, this being a combination of servo-motor and gyroscope. This apparatus has been well tried.